Currently, a promising source of electrons in vacuum devices is field emission of electrons into vacuum from suitable cathode materials. These vacuum devices include flat panel displays, klystrons and traveling wave tubes used in microwave power amplifiers, ion guns, electron beam lithography, high energy accelerators, free electron lasers, and electron microscopes and microprobes. The most promising application is the use of field emitters in matrix-addressed flat panel displays. See, for example, Semiconductor International, December 1991, p. 46; C. A. Spindt et al., IEEE Transactions on Electron Devices, Vol. 38, p. 2355 (1991); I. Brodie and C. A. Spindt, Advances in Electronics and Electron Physics, edited by P. W. Hawkes, Vol. 83, pp. 75-87 (1992); and J. A. Costellano, Handbook of Display Technology, Academic Press, New York, pp. 254 (1992), the disclosures of which are hereby incorporated by reference.
A typical field emission device comprises a cathode containing a plurality of field emitter tips and an anode spaced from the cathode. A voltage applied between the anode and cathode induces the emission of electrons towards the anode. A conventional electron field emission flat panel display comprises a flat vacuum cell, the vacuum cell having a matrix array of microscopic field emitters formed on a cathode and a phosphor coated anode on a transparent front plate. Between cathode and anode is a conductive element called a grid or gate. The cathodes and gates are typically intersecting strips (usually perpendicular strips) whose intersections define pixels for the display. A given pixel is activated by applying voltage between the cathode conductor strip and the gate conductor. A more positive voltage is applied to the anode in order to impart a relatively high energy (400-3,000 eV) to the emitted electrons. See, for example, U.S. Pat. Nos. 4,940,916; 5,129,850; 5,138,237 and 5,283,500, the disclosures of which are hereby incorporated by reference.
A variety of characteristics are known to be advantageous for cathode materials of field emission devices. The emission current is advantageously voltage controllable, with driver voltages in a range obtainable from off-the-shelf integrated circuits. For typical device dimensions (e.g., 1 .mu.m gate-to-cathode spacing), a cathode that emits at fields of 25 V/.mu.m or less is generally desirable for typical CMOS circuitry. The emitting current density is advantageously in the range 0.1-1 mA/mm.sup.2 for flat panel display applications. The emission characteristics are advantageously reproducible from one source to another, and advantageously stable over a very long period of time (tens of thousands of hours). The emission fluctuations (noise) are advantageously small enough to avoid limiting device performance. The cathode is advantageously resistant to unwanted occurrences in the vacuum environment, such as ion bombardment, chemical reaction with residual gases, temperature extremes, and arcing. Finally, the cathode manufacturing is advantageously inexpensive, e.g., no highly critical processes, and adaptable to a wide variety of applications.
Previous electron emitters were typically made of metal (such as Mo) or semiconductor material (such as Si) in nanometer sizes. While useful emission characteristics have been demonstrated for these materials, the control voltage required for emission is relatively high (around 100 V) because of the materials' high work functions. The high voltage operation increases damage caused by ion bombardment and surface diffusion on the emitter tips. High voltage operation also necessitates high power densities to be supplied from an external source to produce the required emission current density. In addition, the fabrication of uniform sharp tips is difficult, tedious and expensive, especially over a large area. The vulnerability of these materials to ion bombardment, chemically active species and temperature extremes is also a serious concern.
Diamond is a useful material for field emitters because of its negative electron affinity and robust mechanical and chemical properties. Field emission devices employing diamond field emitters are disclosed, for example, in U.S. Pat. Nos. 5,129,850 and 5,138,237 and in Okano et al., Appl Phys. Lett., Vol. 64, p. 2742 (1994), the disclosures of which are hereby incorporated by reference. Flat panel displays that employ diamond emitters are disclosed in co-pending U.S. patent application Ser. No. 08/567,867 (our reference Eom 5-118-32-28-26), now U.S. Pat. No. 5,747,918 08/548,720 (our reference Jin 116-30-1), U.S. Pat. No. 5,504,385, U.S. Pat. No. 5,637,950 and U.S. Pat. No. 5,623,180, the disclosures of which are hereby incorporated by reference.
While diamond offers substantial advantages for field emitters, there is a need for diamond emitters capable of emission at yet lower voltages. For example, flat panel displays typically require current densities of at least 0.1 mA/mm.sup.2. If such densities are achieved with an applied voltage below 25 V/.mu.m for the gap between the emitters and the gate, it will be possible for low cost CMOS driver circuitry to be used in the display. Unfortunately, good quality, intrinsic diamond generally does not emit electrons in a stable fashion because of diamond's insulating nature. Therefore, to effectively take advantage of the negative electron affinity of diamond in order to achieve low voltage emission, diamonds need to be conventionally doped into n-type semiconductivity. The n-type doping process, however, has not been reliably achieved for diamond. While p-type semiconducting diamond is readily available, p-doped diamond is not helpful for low voltage emission because the energy levels filled with electrons are far below the vacuum level. For example, a field of more than 70 V/.mu.m is needed for p-type semiconducting diamond to generate an emission current density of 0.1 mA/mm.sup.2. (See, e.g., Zhu et al., J. Appl. Phys., 78, 2707, 1995.)
An alternative method to achieve low voltage field emission from diamond is to grow or treat diamond so that the densities of defects are increased in the diamond structure, as disclosed in U.S. Pat. No. 5,637,950. Such defect-rich diamond typically exhibits a full width at half maximum (FWHM) of 7-11 cm.sup.-1 for the diamond peak at 1332 cm.sup.-1 in Raman spectroscopy, and it is possible for the electric field required to produce an electron emission current density of 0.1 mA/mm.sup.2 from these diamonds to reach as low as 12 V/.mu.m.
Thus, further improved diamond emitter devices, and improved methods for making such devices, are desired, particularly for flat panel displays.